The present invention generally relates to systems for operating hydraulic circuits. More particularly, this invention relates to a hydraulic system for controlling the position of a working (earthmoving) implement on an earthmoving machine, for example, a blade of an excavator.
Compact excavators are an example of multi-functional earthmoving machines that often have multiple standard functions. FIG. 1 illustrates a compact excavator 100 as having a cab 101 mounted on top of an undercarriage 102 via a swing bearing (not shown) or other suitable device. The undercarriage 102 includes tracks 103 and associated drive components, such as drive sprockets, rollers, idlers, etc. The excavator 100 is further equipped with a blade 104 and an articulating mechanical arm 105 comprising a boom 106, a stick 107, and an attachment 108 represented as a bucket, though it should be understood that a variety of different attachments could be mounted to the arm 105. The functions of the excavator 100 include the motions of the boom 106, stick 107 and bucket 108, the offset of the arm 105 during excavation operations with the bucket 108, the motion of the blade 104 during grading operations, the swing motion for rotating the cab 101, and the left and right travel motions of the tracks 103 during movement of the excavator 100. In the case of a compact excavator 100 of the type represented in FIG. 1, the blade 104, boom 106, stick 107, bucket 108 and offset functions are typically powered with linear actuators 109-114, represented as hydraulic cylinders in FIG. 1.
The blade 104 of the excavator 100 and similar earthmoving machines is adapted for moving soil, for example, backfilling a hole or other types of tasks that entail controlling the position of the blade 104 relative to the ground to create a level soil surface, often in spite of changes in machine orientation while driving over uneven ground. In FIG. 1, the blade position is represented as determined by the linear actuators 113 and 114, which may be double-acting, single-rod hydraulic cylinders connected to the blade 104 and the undercarriage 102 of the excavator 100, though it is foreseeable that any number and type of actuators could be used. The flow rate of pressurized oil to the actuators 113 and 114 is typically controlled with a manually-operated hydraulic valve (not shown). This valve commonly includes a position that connects both chambers of the actuators 113 and 114 to each other, allowing the blade 104 to “float” or move freely and follow the contour of the soil surface. The floating function is particularly useful for smoothing soil while driving the excavator 100 backwards and allowing the blade 104 to drag on the ground.
The cylinders that control the blade position of earthmoving machines can also be directly controlled with a hydraulic pump. Several pump-controlled hydraulic systems are known that use constant and variable displacement pumps. If the blade hydraulic system utilizes a variable displacement pump connected to a single-rod actuator in a closed hydraulic circuit, one or more valves typically connect the circuit to a charge pump and compensate for the difference in volume between the two chambers of the actuator resulting from the presence of the rod within one of the chambers. This volumetric compensation may be achieved with a single spool-type valve (such as in U.S. Pat. No. 5,329,767), two pilot-operated check valves, or another way. The floating function described previously can be accomplished in a pump-controlled actuator circuit with the addition of one or more valves for switching pilot lines. In both valve-controlled and pump-controlled circuits, the actuator floating function is achieved by physically connecting the two actuator hydraulic lines together with one or more valves.